1. Field of the Invention
The present invention relates to telecommunication systems and, particularly, to a system that is more robust against in-band interferers.
2. Description of the Related Art
In conventional radio-frequency (RF) receivers, a band-pass filter is used to filter out-of-band interferers. In many cordless and/or wireless systems, the available bandwidth is subdivided into smaller channels. However, the band-pass filter typically has a bandwidth covering the entire available bandwidth.
Such a receiver is illustrated more particularly in FIG. 1. The system 100 includes an antenna 101, a band-pass filter 102, a mixer 104, a voltage-controlled oscillator 106, a band-pass filter 108, a demodulator 110, a low-pass filter 112, and a synchronization block 114.
A modulated signal is received at the antenna 101 and is band-pass filtered by the band-pass filter 102. The band-pass filter 102 reduces the receiving signal bandwidth to the bandwidth that covers all the used channels. By doing so, the band-pass filter 102 filters out the out-of-band interference. The signal output from the band-pass filter 102 is mixed in the mixer 104 with a lower constant frequency signal which may be generated, as shown, by the voltage controlled oscillator 106. The modulated receive signal is thus transferred down to a lower frequency, typically referred to as the Intermediate Frequency (IF). The band-pass filter 108 is provided behind the mixer 104 because the output of the mixer 104 is two down-converted modulated receive signals on two different frequencies, only one of which can be used in the demodulator 110. Thus, only one of the down-converted IF signals is passed through the band-pass filter 108 to the demodulator 112. The demodulator 110 converts the frequency-modulated signal into a baseband signal, which is low-pass filtered using the low-pass filter 112. Finally, the sync block 114 synchronizes to the low-pass filtered signal. For example, the synchronization block may detect one or more synchronization words.
As can be appreciated, when the signal bandwidth is less than the available system bandwidth, the band-pass filter 102 fails to filter out the “out of channel” interferers. These are then mixed and can negatively impact system robustness, which results in a higher bit error rate and voice quality degradation.